March 2j, 19 13] 



NATURE 



87 



-Low velocity type of Ho 



the flow in both air and water 

 being studied at velocities 

 below the critical velocities 

 of the channels used. 



A number of methods for 

 indicating- the direction of 

 motion of the fluids have been 

 tried, and, up to the present 

 time, the best results have 

 been obtained : 



1. In air, by allowing tobacco 

 smoke to issue from a jet at 

 the velocity of the surrounding 

 air stream, on the upstream 



through a channel past a model 

 which is fixed in the channel. 



(b) Those in which the fluid 

 is at rest in the channel, the 

 model being moved relatively 

 to the fluid and channel. 



In both methods great diffi- 

 culties are met with if the 

 velocity of flow be high, owing 

 to the rapid movements of the 

 fluid, but in the first method 

 the fact that in a channel the 

 flow becomes turbulent when 

 the critical velocity is reached 



clocity type of flo 



-High velocity type of flo 



side of the model under 

 investigation. 

 2. Li water: 



(a) Bv coating the model with 

 condensed milk, which is washed 

 off into the eddying regions, 

 making visible the movements 

 nl the fluid in those regions. 



(b) By introducing minute 

 particles of oil (aniline and 

 toluene) of the same density 

 as the surrounding water, the 

 direction of motion of these 

 particles being recorded photo- 



makes observation at high 

 velocities almost impossible. 



During the past two years a 

 research on fluid motion has 

 been in progress at the National 

 Physical Laboratory, and a 

 brief description of some of the 

 experiments which have been 

 described in the report of the 

 Advisory Committee for Aero- 

 nautics may be of interest. 



The Teddington experiments 

 have all been made in the 

 " flowing fluid " type of channel, 



NO. 2265, VOL. 91] 



city type of flow. Wate 



